Process and apparatus for making heat exchanger panels

ABSTRACT

A process and apparatus for forming an as-rolled composite metal panel comprised of at least two sheets of metal having a bond therebetween which can be broken by inflation to form a desired pattern of tubular passageways. The deformation of the sheets of metal during inflation is limited by a maximum aspect ratio. Inflation is provided using an inflation needle having a cross section which is a part or full oval. A fixture is provided for limiting the peeling apart of the composite metal panel upon insertion of the inflation needle.

BACKGROUND OF THE INVENTION

This invention relates to an improved process and apparatus for makingtube in sheet type heat exchange panels. These panels find particularapplication in solar energy collectors and related type devices.

PRIOR ART STATEMENT

Tube in sheet type heat exchange panels have been made commercially formany years by the ROLL-BOND® process as exemplified in U.S. Pat. No.2,690,002 to Grenell. These panels have found wide commercialapplication in refrigerator heat exchangers. More recently they havefound application in the field of solar energy as absorber panels, etc.,as exemplified in U.S. Pat. Nos. 4,021,901 and 4,066,121 to Kleine etal. and U.S. Pat. No. 4,093,024 to Middleton.

The ROLL-BOND® process has proved to be highly effective for thoseapplications, however, there are some difficulties associated with theprocess. One difficulty is the inability to precisely control the finalshape and location of the pattern of stop weld material sandwichedbetween the composite metal sheets after they have been bonded together.The aforenoted difficulties result in the need for large tube freeportions at the sides and ends of the panels, which can greatly reducetheir efficiency and increase the cost of the panels.

While the ROLL-BOND® process as set forth in the Grenell patent hasfound worldwide commercial application, other processes for obtainingtube in sheet type panels are also known. One of those processescomprises an approach wherein the composite metal sheets are "green"bonded together without a stop-weld pattern and the tubular pattern isformed by inflation in a die whose cavity defines the tubular pattern.Various examples of this process are set forth in U.S. Pat. Nos.3,271,846 to Buechele et al., 3,346,936 to Miller et al., 3,435,504 toMiller and 3,465,568 to Jonason and in Australian Pat. No. 212,814 toWatson.

In the Australian patent to Watson there is disclosed a process formaking heat exchangers wherein clean faced sheets of metal are rollforged to provide a bond that can be broken by internal pressure. Thebonded sheets are placed in a matrix die having a cavity or cavitiestherein corresponding to the shape and configuration of the ductsrequired in the heat exchanger. Pressure is applied to inflate therespective ducts, etc., in conformity with the die cavity. The processas disclosed can be carried out without a bond enhancement heattreatment.

The Miller patent describes the use of the bonding process employingcold rolling as disclosed in U.S. Pat. Nos. 2,691,815 and 2,753,623 toBoessenkool et al. Another known bonding process wherein sheets arestrongly bonded together by cold rolling is disclosed in U.S. Pat. No.3,397,045 to Winter.

Conventionally cylindrical inflation needles are inserted into thecomposite metal panels formed as described above to providecommunication with a source of fluid under pressure for inflating thepassageways in the panel. If one attempts to employ such a cylindricalneedle with a composite in its rolled temper after bonding a criticalaspect ratio is exceeded which can result in fractures, etc. of themetal around the inflation needle. Further, if the inflation needle soinserted into the composite metal panel while the panel is unrestrained,excessive peeling apart of the sheets making up the panel can occur.

SUMMARY OF THE INVENTION

In accordance with this invention an improved process and apparatus isprovided for making heat exchanger panels particularly adapted for usein solar applications. Clean faced sheets of metal are bonded togetherby cold rolling to a desired reduction in thickness to form a compositehaving a "green" bond which can be broken at reasonable inflationpressures. The inflation is carried out while the composite is clampedwithin a die having a cavity or cavities defining a desired tubularconfiguration.

The process and apparatus of the present invention is particularlyadapted for use with copper and copper alloys or aluminum and aluminumalloys. For copper and copper alloys, it has been found that the totalreduction in thickness for bonding by cold rolling must be maintainedwithin a specific critical range of reduction in order to insure thatthe composite can be later expanded at convenient pressures by dieinflation.

It is preferred in accordance with this invention that the die inflationbe carried out with the composite panel in its roll temper followingbonding. The as rolled condition of the composite panel requires thatthe shape of the passageways must be controlled and the aspect ratio ofthe passageways must be maintained below a desired level in order thatthe tube walls do not rupture while still obtaining satisfactory heatexchange panels by die inflation.

In accordance with the present invention the inflation needle insertedinto the panel prior to die inflation has an oval or half-oval crosssection so that it can be inserted into the composite panel in the rolltemper without causing fracture or other defects. Further, a uniquefixture is provided in accordance with this invention for preventingexcessive peeling apart of the sheets of the panel when the inflationneedle is inserted therein.

The process and apparatus of this invention is suited for making panelswhich in the "green" bonded condition have sufficient bond strength foruse in conventional low pressure applications such as solar energyabsorber plates. If the panels are desired for use with higher pressureapplications, it is possible in accordance with this invention toprovide bond enhancement by heat treating the panels at a desiredtemperature, after the inflation of the tubular passageways therein.

The apparatus for carrying out the process of this invention includes aninflation needle having an oval or half-oval for configuration forproviding communication with a source of fluid under pressure forinflating the tubular passageways therein. A fixture for inserting theneedle also forms part of the invention.

Accordingly, it is an object of this invention to provide an improvedprocess and apparatus for making heat exchanger panels.

These and other objects will become more apparent from the followingdescription and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a heat exchanger panel inaccordance with this invention;

FIG. 2 is a schematic illustration of an apparatus in accordance withthis invention for continuously "green" bonding strip in coil form;

FIG. 3 is a schematic illustration of an apparatus in accordance withthis invention for dividing the coiled composite strip into panel lengthsegments;

FIG. 4 is a perspective view of a metal sheet used to make a compositepanel in accordance with a different embodiment of the presentinvention;

FIG. 5 is a perspective view of overlapped and staked sheets prior tobonding;

FIG. 6 is a schematic representation of an apparatus for bonding thesheets of FIG. 5 in accordance with this invention;

FIG. 7 is a schematic representation of a die inflation apparatus inaccordance with this invention;

FIG. 8 is a partial cross-sectional view of the die inflation apparatusof FIG. 7, along the line 8--8 in FIG. 7;

FIG. 9 is a partial perspective view showing an inflation needleinsertion fixture in accordance with this invention.

FIG. 10 is a partial cross-sectional view of a passageway in a heatexchanger panel of this invention;

FIG. 11 is a perspective view of an inflation needle in accordance withone embodiment of this invention;

FIG. 12 is a perspective view of an inflation needle in accordance witha different embodiment of this invention; and

FIG. 13 is a perspective view in partial cross-section of a coolantapplication chamber.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now to FIG. 1 there is shown by way of example a schematicillustration of a heat exchanger panel 10 useful in applicationsinvolving solar energy. For example, the panel 10 is useful as anabsorber plate in a solar energy collector. The panel shown in FIG. 1 isrepresentative of a plank type panel which is approximately about 12inches wide and from 6 to 10 feet long. Absorber panels 10 havingvarious sizes are known in the art. The panel 10 itself is of a tube insheet configuration. The panel is formed from composite metal sheetswhich remain bonded together everywhere except where tubular passageways11 or channels have been expanded within the composite. A variety oftechniques have been utilized to form tube in sheet type heat exchangerpanels as described in the background of the invention. The presentinvention is particularly directed to an apparatus and process forforming a tube in sheet heat exchanger as in FIG. 1 by first "green"bonding the composite sheets together followed by die inflation.

The specific design of the panel 10 shown in FIG. 1 includes entry 12and exit ports 13 communicating with triangular shaped headers 14. Theheaders 14 include a plurality of bonded islands 15 for obstructing anddirecting the flow of the heat exchange fluid to provide uniform flowacross the width of the panel 10. The triangular shaped headers 14 areconnected together by a plurality of parallel tubular passageways 16.

Difficulties arise in the manufacture of panels 10 of the general typeas shown in FIG. 1 or any of the various alternative configurationswhich are known in the art, when the "green" bond and die inflationmethod is used. The inflation pressure necessary to form the tubularpassageways 11 in the composite sheet while supported in the die is adirect function of the bond strength between the sheets of thecomposite. It will be shown hereinafter that the bond strength is afunction of the percent reduction employed in bonding the compositetogether by rolling and, further, that heating the as-bonded compositeat elevated temperatures results in significant bond enhancement.

The present process is particularly directed to the use of cold rollingas the means for bonding the composite sheets together prior to dieinflation. The range of reduction utilized to obtain the desired boundlevel causes a substantial residual heat build up in the panel so thatthe panel exits the rolls at an elevated temperature. The panels arenormally stacked after bonding and depending on the number of panels inthe stack the rate at which they cool to room temperature varies widely.The process of this invention is also useful in connection with theformation of the composite panel in a continuous manner through the useof coiled metal strip which after bonding may be recoiled and then cutto the desired panel lengths. When the composite strip is coiled afterbonding the tightness of the wrap of the coil will affect how fast theresidual heat built up in the metal during bonding is dissipated,thereby resulting in a variation in the rate of heat extraction from thecomposite metal after bonding. This variation in the heat extractionrate is believed to cause an undesirable variation in the bond strengthof the composite metal because the elevated temperature of the as-bondedcomposite can provide a degree of bond enhancement. The smaller the ratethat the residual heat is extracted from the panel the greater thedegree of bond enhancement and vise a versa.

To overcome these problems in accordance with the present invention thecomposite metal is quenched or otherwise quickly cooled to a temperaturebelow which bond enhancement does not occur immediately following thebonding operation. In this way, the degree of bonding of the compositemetal sheets can be accurately controlled by the percent reduction towhich the sheets are subjected during cold rolling and will besubstantially unaffected by the number of panels stacked together whendiscrete panels are manufactured or the tightness of the wrap of thecoil when a continuous composite strip is made.

Referring now to FIG. 2 the process and apparatus in accordance with oneembodiment of the invention will be described. This embodiment isdirected to the formation of a continuous coil 20 of composite metal. Acontinuous strip comprising one sheet 21 of the composite is fed from afirst coil 22 and a continuous strip comprising the opposing sheet 23 ofthe composite is fed from a second coil 24. The contacting faces of therespective sheets 21 and 23 are wire brushed and otherwise suitablycleaned prior to meeting at the nip defined by the upper 25 and lower 26rolls of the rolling mill 27. Since the cold rolling for bonding isnormally carried out at high speeds the contacting faces of the sheets21 and 23 are preferably cleaned "off-line", namely the coiled strip 22and 24 is cleaned prior to being payed off into the nip of rolls 27. Ifan appropriate high speed cleaning method were available then the sheets21 and 23 could be preferably cleaned "in line" between the coils 22 and24 and the rolls 27. The rolling mill 27 comprising rolls 25 and 26subjects the overlapped sheets 21 and 23 to a desired reduction inthickness to provide a so-called "green" bonded composite 28 having amoderate degree of bond strength.

Immediately adjacent to the rolling mill 27 and in close proximitydownstream thereof, there is positioned a coolant application station 29wherein coolant is applied to the as-bonded composite metal strip 28 inorder to reduce its temperature to a temperature below which substantialbond strength enhancement will not occur. Following the quenchingoperation the composite metal strip may be again coiled for furtherprocessing as desired. The coolant application station 29 shown isarranged preferably above the strip 28 and directs a curtain of acoolant such as water against the top surface of the composite strip 28.The rate of coolant application may be set as desired and will vary withthe composition of the composite strip 28, its thickness and its rate oftravel through the station 29. The rate of coolant application should besufficient to lower the temperature of the strip below the temperaturerequired for bond enhancement.

If desired, an auxiliary coolant application station 29' could belocated below the strip 28 to apply the coolant to the bottom surface ofthe strip. Alternatively the coolant application station 29' could beused in place of the station 29.

After bonding the composite strip 28 may be coiled as in FIG. 2 forfurther processing off-line or it could be subjected to furtherprocessing in line as desired.

Referring now to FIG. 3 the coil 20 of composite metal strip 28 fromFIG. 2 is fed to a roll straightener 30 or other suitable straighteningdevice and then to a shearing station 31 wherein unexpanded compositemetal panels 32 having a desired length are severed from the strip. Ifdesired, the shearing operation can be carried out in line in theapparatus of FIG. 2 in place of the coiling operation shown therein.This could be accomplished through the use of a flying shear as areknown in the art.

Alternatively, in accordance with another embodiment of this inventionthe composite metal panels 32 may be formed from panel size strips ofmetal 40 such as shown in FIG. 4. Two such strips 40 and 40' having atleast one face brushed or otherwise suitably cleaned are placed inoverlapping relationship as in FIG. 5 with the cleaned faces inengagement. They are staked 41 together at the corners of the panel inaccordance with conventional practice. The staked sheets 40 and 40' arethen fed as in FIG. 6 through the nip of a rolling mill 27 as in theprevious embodiment to provide the desired "green" bond. Immediatelyafter bonding as in the previous embodiment the composite metal panel 32is cooled to a temperature at which noticeable bond enhancement will notoccur for the particular materials being bonded. Since in thisembodiment of the invention the original sheets were cut to a desiredsize to yield after rolling a desired size panel the shearing operationas described by reference to FIG. 3 is not required.

At this point, the panels 32 manufactured by the process described byreference to FIGS. 2 and 3 or the process described by reference toFIGS. 4, 5 and 6 are placed between die plates 50 and 51 as in FIGS. 7and 8. The die plates 50 and 51 are clamped by conventional means suchas press 53 and 54 to tightly seal the composite between them. At leastone of the die plates includes a cavity 52 defining the desired patternof tubular passageways 11 in the heat exchange panel 10. In theembodiment shown in FIGS. 7 and 8 only one of the die plates 50 and 51includes such a cavity 52 so that the tubular passageways 11 whichresult will have one side flat. While only one die 50 has a cavity inthe embodiments shown, if desired the opposing die 51 can have a cavityin correspondence with the cavity of the die 50 to provide tubularpassageways 11 expanded out from both sides of the panel 32.

The tubular passageways are formed in the composite panel 32 by theapplication of fluid pressure. This is accomplished by inserting aninflation needle 60 into an edge 61 of the panel in communication withthe desired tubular passageway pattern. Air or other suitable fluid isthen forced into the composite panel to break the "green" bond betweenthe sheets 40 and 40' or 21 and 23 in the region of the die cavity 52where the sheets are not clamped together. Continued pressure deformsthe sheets into conformity with the die cavity 52 thereby forming thedesired passageway pattern in the composite panel 32. Suitable inflationpressures comprise from about 1000 to about 4000 psi. Inflation can becarried out pneumatically or hydraulically, however, pneumatic inflationis preferred.

Referring now to FIGS. 9 through 12 further details will be givenconcerning the inflation needle 60 arrangement in accordance with thisinvention. The composite panels 32 in accordance with this inventionprior to inflation are in their full rolled temper. Therefore, the metalis comparatively hard from cold working. The panels 32 in accordancewith this invention can be expanded in their as-rolled temper to providethe desired passageway configuration without treatment or softening ofthe composite metal. This is accomplished by controlling the tube heightand tube cross section within certain critical limits. In particular, anoval or half-oval tube configuration 63 has been found to be required.

In FIG. 10 a typical tube 11 cross section is shown. The aspect ratio ofthe inflated tube 11 comprises the peak height "h" of the tube walldivided by the width "W" of the tube. It has been found in accordancewith this invention that the aspect ratio is critical when one attemptsto inflate the composite metal panel 32 in the rolled temper. For copperAlloy C 11,000 an aspect ratio of less than about 0.24 is required inorder to avoid failures in the inflated tubes 11 such as necked regionsin the tube wall or fractures in the tube wall. It is preferred inaccordance with this invention that the aspect ratio be maintained at alevel less than about 0.2 and most preferably less than about 0.15.Below 0.15 it is unlikely that a failure would occur in the tube wall ofan inflated panel in accordance with this invention formed from acomposite wherein the expanded tube wall is comprised of Alloy C 11,000.

For other metals and alloys such as aluminum and aluminum alloys theaspect ratio may vary and is very much a function of the ductility ofthe metal in the rolled temper. An aspect ratio for copper or copperalloys of less than 0.15 appears to be acceptable in the light of thesuccess of various commercially available copper solar panels sold byOlin Corporation under the name SOLAR-BOND® which are formed by theROLL-BOND® process, since those panels have conventionally used aspectratios of 0.14. It is possible in accordance with this invention toutilize die cavities 52 having sharp edges defining the edges of thetubular passageways as in FIG. 10 if one maintains the aspect ratio ofthe inflated tubes below the 0.15 limit. If those edges of the diecavity are rounded, then higher aspect ratios within the limitsdescribed above can be utilized.

Conventionally for inflating tube in sheet type composite metal heatexchange panels cylindrical inflation needles have been used. In view ofthe critical limits imposed upon the expansibility of the metal in theroll temper composite panels 32 of this invention it has been determinedthat the inflation needle must have an oval cross section so as to onlydeform the sheets 40, etc., within the critical limits of their aspectratio. For a one side flat type configuration of the tubular passageways11 the inflation needle 60 can have a D-shaped or half-oval crosssection as in FIG. 11. For a two side inflated tubular passageway 11wherein the passageway is distended into both sheets of the compositepanel 32, the inflation needle 60' would have the full oval shape as inFIG. 12. Each inflation needle 60 or 60' is hollow and includes apassageway A or B, respectively, which communicates between the source(not shown) for applying the inflation fluid under pressure to the panel32 and the interior of the panel itself.

In order to insert the inflation needle 60 or 60' into the panel 32without unnecessarily peeling the composite metals apart a uniquefixture has been devised as shown in FIG. 9. The fixture comprises twoplates 64 and 65 clamped against opposing faces of the composite panel32 at the edge 61 where the inflation needle is to be inserted. For aD-shaped inflation needle 60 one of the plates includes a U-shaped slot66 extending in the direction of the inlet passageway 12. If one wereemploying the oval needle 60' of FIG. 11, then both plates 64 and 65would include such a U-shaped slot 66. The U-shaped slot defines anopening into which the respective sheet 40 or 40' or 21 or 23 of thecomposite panel 32 can be expanded for inserting the inflation needle 60or 60'. Since the two plates 64 and 65 are clamped together, theU-shaped slot serves to limit the total area of the composite panel 32which can be separated or peeled apart. As in accordance withconventional practice, the composite panel 32 at the edge 61 adjacentthe U-shaped slot 66 is pried apart using a chisel. When it issufficiently pried apart, the inflation needle 60 or 60' is pushed intothe pried apart opening to a desired extent.

When the panel 32 with the inflation needle is inserted into the dies 50and 51 of FIGS. 7 and 8, the die cavity 52 closes down about theinflation needle 60 or 60' inserted in the panel to seal the sheets tothe needle for the inflation operation. The plates 64 and 65 used forinsertion of the needle 60 or 60' are, of course, not used during theactual inflation operation, but rather the die 50 and 51 itself clampsthe composite sheets in the region of the inflation needle.

Referring now to FIG. 13 a coolant application apparatus 70 which couldbe used as the coolant application station 29 in either FIGS. 2 or 6 isshown. The coolant application station 70 comprises: a manifold 71; aninlet port 72; a discharge slot 73 and a baffle 74. The manifold is inthe form of a rectangular six sided box. The end which is shown open inthe section of FIG. 13 would be sealed by a suitable end plate (notshown). Water or other suitable coolant is pumped from a suitable sourcenot shown into the inlet port 72. The water is uniformly distributed bythe baffle 74 and fills the chamber defined by the manifold 71. Thewater is discharged as a water curtain onto the strip through thedischarge slot 73.

The coolant application station 29 or 29' in accordance with the presentinvention should for copper or aluminum or alloys of copper or aluminumcool the composite panels 32 or strip 28 to a temperature preferablybelow about 200° C. and in particular to a temperature below the onsetof recrystallization of the metal of the composite. In this way, bondenhancement will not occur to any effective extent. The coolant can beapplied in any desired manner such as the water curtain as describedabove or as a spray. If a spray is used, it need not be at a very highpressure since it is not necessary for the water spray to pierce aboundary layer of steam as occurs in the prior art when spray quenchinghot rolled metal.

The "green" bond in accordance with this invention as described above ispreferably formed by cold rolling the strips 21 and 23 or sheets 40 and40' together. This may be accomplished in a single pass through therolling mill 27 or in multiple passes through the rolling mill asdesired. The particular bonding process preferred in accordance withthis invention is essentially that disclosed in the above-identifiedAustralian patent to Watson. For copper and copper alloys it has beenfound that the bonding when carried out in a single pass should beaccomplished by a total percent reduction in thickness of from about 45%to about 75% and preferably from about 60% to about 75%. If desired forshape or gage control, the bonding operation can be carried out in twoor more passes wherein the percent reduction in thickness for the firstpass is at least about 45% and the percent reduction in thickness in thesecond pass is up to an amount such that the total reduction inthickness of the composite strip is from greater than about 70% to lessthan about 85%. The total percent reduction in thickness of theoverlapped sheets 21 and 23 or 40 and 40' comprises the originalthickness of the overlapped sheets less the final thickness of theuninflated composite strip 28 or panel 32 divided by the originalthickness of the overlapped sheets.

It has been found that it reductions in thickness in excess of theaforementioned ranges are utilized the composite metal strip 28 orpanels 32 will be too strongly bonded together for them to be separatedat reasonable inflation pressures. The bond strength of the compositeincreases rapidly to a level at which the bond is tronger than themetals of the composite, if the aforenoted ranges of reduction inthickness are exceeded. On the other hand, if too low a reduction inthickness is taken the composite metal sheets will not be sufficientlystrongly bonded together to avoid being peeled apart or otherwiseseparated in use. It has been found in accordance with this inventionthat the bond strength provided by a reduction in thickness within theaforenoted ranges for copper and copper base alloys is sufficient toallow the as-inflated panel 10 to be used as a solar energy absorberpanel 10 in low pressure applications without further bond enhancement.

For other alloy systems such as aluminum or aluminum alloys the percentreduction in thickness necessary to obtain the desired "green" bond willvary depending on the alloy and may be determined by routineexperimentation but will always be in the range of 30 to 70%.

Panels of high copper alloys bonded within the ranges asabove-identified possess a sufficient bond strength as described abovethat they will not peel apart when handled or in ordinary applicationsinvolving low pressure heat exchange fluids. If it is desired, however,that the bond strength of the panels after inflation be increased toinsure their integrity and enable their use at higher working pressuresthis can be accomplished following inflation by a diffusion heattreatment which serves to enhance the bond strength of the compositepanel. For copper and high copper alloys a suitable treatment wouldcomprise holding the panels for at least about 1/2 hour at a temperatureof at least about 400° C. For higher temperatures shorter times can beemployed and visa versa. High pressure solar applications are thosewherein the panels 10 must withstand potable water line pressure inregular service of approximately 60 psi and a test pressure of 150 psi.Low pressure applications are those in which operating pressures are onthe order of 20 psi and test pressures in the 50 to 60 psi range.

"Green" bond as the term is used herein refers to a bond of moderatestrength which will not break in ordinary handling but which will breakat inflation pressures within the abovenoted range of from about 1000 toabout 4000 psi. Bond enhancement as the term is used herein refers to anincrease in bond strength after completion of the bonding operation,e.g., cold rolling. Bond enhancement is a kinetic process dependent ontemperature and time at temperature, with the temperature being far moresignificant than the time.

The process and apparatus of this invention are applicable to a widerange of metals and alloys including copper and copper alloys, aluminumand aluminum alloys as noted above as well as iron and iron alloys andthe alloys of other engineering metals. Particularly preferred metalsand alloys include high copper alloys for good conductivity andcupro-nickel alloys for corrosion resistance.

The patents which are set forth in this application are intended to beincorporated by reference herein.

It is apparent that there has been provided in accordance with thisinvention a process and apparatus for making heat exchanger panelstherefrom which fully satisfies the objects, means and advantages setforth hereinbefore. While the invention has been described incombination with specific embodiments thereof, it is evident that manyalternatives, modifications and variations will be apparent to thoseskilled in the art in light of the foregoing description. Accordingly,it is intended to embrace all such alternatives, modifications andvariations as fall within the spirit and broad scope of the appendedclaims.

What is claimed is:
 1. In an apparatus for forming a heat exchangerhaving a desired pattern of inflated tubular passageways from anas-rolled composite metal panel in the absence of a stop weld patterndefining said tubular passageways, said panel comprising at least twosheets of metal having a bond therebetween which can be broken byinflation; said apparatus comprising:an inflation needle for insertionbetween said sheets of said composite, and means for receiving saidpanel with said inflation needle inserted therein and for inflating saiddesired pattern of tubular passageways in said panel, said inflationmeans including at least one die having a cavity defining said tubularpassageways; the improvement wherein: means are provided for preventingexcessive peeling-apart of said sheets of said composite metal panelupon insertion of said inflation needle therein, said peeling-apartpreventing means being operative upon said panel before it is receivedby said tubular passageway inflating means, said peeling-apartpreventing means including: a first clamping member and a secondclamping member arranged to clamp said composite metal paneltherebetween at an edge thereof wherein said inflation needle is to beinserted; at least one of said clamping members including a slotdefining an opening immediately adjacent to said panel of limited extentand extending inwardly of said panel from an edge thereof, said panel inan area defined by said slot being free to expand while in an areaclamped by said members being restrained from exapnsion; whereby uponinserting of said inflation needle between said sheets of said panel, inalignment with said slot, the sheet of said panel which engages said atleast one of said clamping members expands only in said area defined bysaid slot.
 2. An apparatus as in claim 1 wherein both of said membersinclude a said slot.
 3. An apparatus as in claim 1 wherein saidinflation needle has an oval cross section.
 4. An apparatus as in claim1 wherein said inflation needle has a D-shaped cross section.
 5. Anapparatus as in claim 1 wherein said sheets of metal in said compositeare deformable by said inflation to a limited extent without fracturingto form said desired pattern of tubular passageways, said limited extentof deformation defining a maximum aspect ratio for said tubularpassageways, and wherein said inflation needle is sized so that uponinsertion into said panel it will not deform said sheets of metal inexcess of said maximum of said aspect ratio.
 6. An apparatus as in claim5 wherein said tubular passageways have a cross section which is atleast part oval and wherein said cross section of said inflation needlecorresponds to a shape of the inside of said tubular passageways.
 7. Anapparatus as in claim 5 wherein said sheets of metal comprise copper ora copper base alloy and wherein said maximum aspect ratio comprisesabout 0.24.
 8. An apparatus as in claim 1 wherein said first and secondclamping members comprise blocks of metal.
 9. In a process for forming aheat exchanger having a desired pattern of inflated tubular passagewaysfrom an as-rolled composite metal panel in the absence of a stop weldpattern defining said tubular passageways, said panel comprising atleast two sheets of metal having a bond therebetween which can be brokenby inflation; said process comprising:inserting an inflation needle intosaid composite metal panel between said sheets; and following theinsertion of said inflation needle placing said panel with saidinflation needle inserted therein into an inflation press including atleast one die having a cavity defining said tubular passageways, andinflating said desired pattern of tubular passageways in said panel; theimprovement wherein: said step of inserting said inflation needle intosaid composite metal panel comprises: preventing excessive peeling-apartof said sheets of said composite metal panel upon insertion of saidinflation needle, said step of preventing excessive peeling-apart ofsaid sheets, comprising the steps of: providing a first clamping memberand a second clamping member arranged to clamp said composite metalpanel therebetween at an edge thereof wherein said inflation needle isto be inserted, at least one of said clamping members including a slotdefining an opening immediately adjacent to said panel of limited extentand extending inwardly said panel from said edge thereof; and insertingsaid inflation needle between said sheets of said panel in alignmentwith said slot; whereby the sheet of said panel which engages said atleast one of said clamping members expands only in an area defined bysaid slot.
 10. A process as in claim 9 wherein said sheets of metal insaid composite metal panel are deformable by said inflation to a limitedextent without fracturing to form said desired pattern of tubularpassageways, said limited extent of deformation defining a maximumaspect ratio for said tubular passageways, and wherein said sheets ofmetal comprise copper or copper base alloys and said maximum aspectratio is about 0.24.